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262                     METALLURGY   OF   CAST   IRON.
In making chilled work, it is essential to understand the various effects which the different metalloids have-in controlling the combined carbon, associated with a knowledge of the individual effect of each metalloid in regulating the character of the hardness best calculated to stand the wear of friction or heat, as outlined in the former part of this Chapter.
In a general way it can be said that the percentage of the chemical constituents which combine to make chill castings ranges in silicon from 0.50 to, manganese from 0.55 to 1.50 per cent., phosphorus from 0.20 to 0.70 and in sulphur from .02 to .10, with the total carbon from 2.50 to 3.75.
The quality to be first understood is the depth of the chill and hardness desired in a casting; second, the chilling properties of the iron to be used. To make a comparative test in order to learn of the chilling qualities of an iron by casting chill specimens, it should be remembered that 4t hot iron" will chill deeper than "dull iron," and that note should be taken of the same, in connection with the other elements of chilling, as outlined in Chapter LVI. It is also to be remembered that manganese will give longer life to the fluidity of metal than sulphur, where preference can be given either, in producing the combined carbon. It is very important in assisting to prevent " cold shuts" or " chill cracks," when pouring a mould, to have the metal run freely, and hence the advantage of manganese over sulphur, as above stated.*
* Information on the thickness of chills, methods for making and pouring "chilled" castings, also making clean and smooth and perfect work, can be found on pages 272 and 276 in "American Foundry Practice," and page 234 in "Moulder's Text-Book."from the white to the grey in chilled castingsly the same depth.... Shop scrap     ......	15            x ro           x	1.50 i. So	22.50 iS.oo